1. Field of the Invention
The present invention relates to a high-resistance high-zirconia cast refractory material suitable for glass melting furnaces. More particularly, the present invention relates to a high-resistance high-zirconia cast refractory material excelling in thermal cycle stability and free from peeling during heating (at about 500° C.) and also exhibiting a remarkably high electric resistance at high temperatures.
2. Description of the Related Art
Among conventional common refractory materials for glass melting furnaces is a cast refractory material rich with ZrO2 (zirconia or zirconium oxide). This is because ZrO2 is a metal oxide having a high resistance to corrosion by molten glass. An example of such cast refractory materials is a high-zirconia cast refractory material containing no less than 80 wt % ZrO2.
The high-zirconia cast refractory material exhibits good corrosion resistance for molten glass of any kind on account of its high ZrO2 content and its compact structure. In addition, it causes no defects (such as stone and cord) to molten glass because it forms no reaction layer on its interface with molten glass. Consequently, the high-zirconia cast refractory material is suitable for production of high-quality glass.
The high-zirconia cast refractory material has a mineral composition consisting mainly of zirconia crystals of monoclinic system, with their grain boundaries being filled with a small amount of glass phase.
On the other hand, the zirconia crystals are known to undergo reversible transformation in crystal system (between monoclinic and tetragonal) accompanied by a steep volume change at about 1150° C. The volume change due to transformation generates stresses, but they are relieved as the glass phase flows. This permits regular production of a high-zirconia cast refractory material free from cracking in the casting process. However, the high-zirconia cast refractory material containing a small amount of glass phase greatly varies in its characteristic properties depending on the amount and kind of the constituents of the glass phase.
Glass is usually composed of the following constituents, which are classified into three groups.
Oxides such as SiO2, B2O3, and P2O5, which vitrify by themselves. They are referred to as glass-forming oxides or “glass former”. SiO2 glass forms a network structure consisting of Si—O—Si linkages.
Alkali metal oxides such as Na2O and alkaline earth metal oxides such as CaO, which are referred to as glass modifying oxides or “glass modifier”. They easily enter the interstices of the network structure.
Other oxides such as Al2O3 and TiO2, which have the intermediate properties of the above-mentioned oxides. They are referred to as intermediate oxides or “intermediate”. Intermediate acts as either glass former or glass modifier.
The network structure consisting of glass-forming oxides varies depending on the modifying oxides. In other words, glass will vary in its characteristic properties, such as viscosity, transition temperature, and electric resistance, depending on the amount of modifying oxides (or the ratio of Former to Modifier).
Meanwhile, alkali-free glass for liquid crystal panel (LCD) needs a higher electric resistance than conventional one for its improved performance. Therefore, it should be produced by using a melting furnace lined with a high-zirconia refractory material which has a high electric resistance.
However, the conventional high-resistance refractory material is uncertain about its electric resistance which is measured as soon as a predetermined temperature has been reached or after standing for several hours at a predetermined temperature. The value of electric resistance measured in this manner apparently lacks stability and consistency.
That is, a sample of high-zirconia cast refractory material may increase in electric resistance after standing for a long time. To be specific, the electric resistance measured after standing at 1500° C. for 12 hours equals 160% of the value measured immediately after heating to 1500° C. This is due to precipitation of zircon in glass phase or precipitation of zircon (with a high electric resistance) around zirconia crystals (with a low electric resistance), which results in the high-zirconia cast refractory material increasing in electric resistance.
Precipitation of zircon helps increase electric resistance as mentioned above but causes cracking and powdering during thermal cycle as mentioned later; therefore, it is not desirable for high-zirconia cast refractory materials.
The foregoing has aroused a demand for a high-zirconia cast refractory material that maintains a high electric resistance stably at high temperatures.
The fact that conventional high-zirconia cast refractory materials constituting a glass melting furnace sometimes chip off at corners or peel off (conchoidally) from the furnace inside at the time of furnace heating has also aroused a demand for a high-zirconia cast refractory material that remains stable without peeling during heating.
Once a high-zirconia cast refractory material is damaged, damaged parts are extremely vulnerable to corrosion by molten glass. This poses a problem with such defects as stone and cord in molten glass.
It is known that peeling that occurs at the time of heating mostly arises from residual stress and pitlike defects present in the surface of the product. Residual stress in the product may be compressive stress or tensile stress.
Compressive stress is defined as a convergent force toward a point in the refractory material and tensile stress is defined as a divergent force from a point in the refractory material.
In general, a refractory material expands in its surface upon heating, thereby giving rise to a compressive stress opposite to expansion. This compressive stress combines with residual stress (which may be compressive one) to produce a force that acts on the surface of the high-zirconia cast refractory material. This force is large enough to cause peeling at the time of heating even though the residual stress is comparatively small. Consequently, the residual stress should preferably be as small as possible and be tensile one rather than compressive one.
Refractory products usually have pitlike surface defects that occur when the melt is cast into a mold. Such defects also cause peeling at the time of heating.
When acted on by the resulting force of residual stress and stress due to heating, the part adjacent to pitlike defects, which is weaker than the compact part, may peel off at the time of heating.
In fact, pitlike defects are often found in the part where peeling has occurred at the time of heating.
Most glass melting furnaces constructed of high-zirconia cast refractory materials are of burner combustion type. And, such furnaces are run, with burners switched at intervals of tens of minutes. The switching of burners raises or lowers the temperature of the surface of the cast refractory materials. This means that the cast refractory materials, which are used for several years, undergo heating cycles repeatedly. This is the reason why there has been a demand for a high-zirconia cast refractory material which remains stable to heating cycles.
It is important for stability toward heating cycles that the glass phase to absorb an abrupt volume change of zirconia crystals at about 1150° C. is not affected by heating cycles. The glass phase with zircon precipitating therein is unable to absorb the volume change of zirconia, and zirconia that has undergone heating cycle tests has a large permanent expansion coefficient. This results in the refractory material suffering cracking and powdering. There is the following relationship between stability of the glass phase and the permanent expansion coefficient measured after heating cycle tests.
The permanent expansion coefficient of the high-zirconia cast refractory material which has undergone heating cycle tests exceeds 10% if zircon precipitates in the glass phase, whereas it is no more than 10% if the glass phase remains stable without zircon precipitation.
Therefore, the high-zirconia cast refractory material should preferably have a permanent expansion coefficient no more than 10% after heating cycle tests.
The fact that part of the high-zirconia cast refractory material in which the glass phase is replaced by LCD glass has a permanent expansion coefficient of 3% to 7% after heating cycle tests suggests that good stability will be gained if the permanent expansion coefficient is identical in the high-zirconia cast refractory material and that part of the high-zirconia cast refractory material in which the glass phase is replaced by LCD glass.
It follows, therefore, that the high-zirconia cast refractory material should preferably have a permanent expansion coefficient no more than 5% after heating cycle tests.
Refractory materials having a high electric resistance are disclosed in Japanese Patent Laid-open Nos. Sho-63-285173, Hei-4-193766, Hei-8-48573, Hei-8-277162, and Hei-10-59768, and WO2005/068393.
Heat cycle stability is described in Japanese Patent Laid-open Nos. Hei-4-193766, Hei-8-48573, and Hei-8-277162. Prevention of surface peeling at the time of heating is described in Japanese Patent Laid-open Nos. Hei-8-48573 and Hei-8-277162.
The refractory material proposed in Japanese Patent Laid-open No. Sho-63-285173 is a high-resistance high-zirconia refractory material which contains at least one species of K2O, SrO, BaO, and Cs2O in an amount being 1.5 wt % or less in place of Li2O, Na2O, CaO, CuO, MgO, and P2O5 which have a small ionic radius. This refractory material has a high electric resistance but does not contain CaO which is required to stabilize the glass phase. In addition, because of the absence of CaO, it has a large tensile stress and easily cracks when heated on one side.
The refractory material proposed in Japanese Patent Laid-open No. Hei-4-193766 is a high-zirconia electrocast refractory material which has a high electric resistance and is stable to heating cycles. It contains 1-3 wt % of Al2O3, 0.3-3 wt % of at least one species of BaO, SrO, and CaO, and 0-1.5 wt % of ZnO, and it does not contain Na2O and K2O.
This refractory material, however, does not have a sufficiently high electric resistance because of the high content of Al2O3. Moreover, it is poor in heat cycle stability because of the lack of Na2O and K2O.
The refractory material proposed in Japanese Patent Laid-open No. Hei-8-48573 is a high-zirconia electrocast refractory material which has a high electric resistance and is little vulnerable to surface peeling, with good stability toward repeated heating (or heating cycles). It contains more than 0.05 wt % of Na2O and 0.05-3 wt % of BaO, SrO, and MgO in total.
This refractory material, however, has a stable glass phase but does not have a sufficiently high electric resistance on account of the high content of Na2O exceeding 0.05 wt %.
In addition, it has a large permanent expansion coefficient, which is detrimental to heat cycle stability, when it contains BaO (alkaline earth metal oxide) in a large amount close to the upper limit (3 wt %).
It is claimed in the disclosure to have a surface residual stress being 80 MPa or smaller (as tensile force) and 50 MPa or smaller (as compressive force). Because of such a broad range of stress, it is subject to peeling at the time of heating if it has pitlike defects in its surface.
The refractory material proposed in Japanese Patent Laid-open No. Hei-8-277162 is a high-resistance high-zirconia electrocast refractory material which contains 0.05 wt % or more of Na2O, 0.05-1 wt % of Na2O and K2O in total, 0.05-3 wt % of BaO and MgO in total, and 0.2 wt % or less of P2O5. It is stable to repeated heating (heat cycles) and is less subject to surface peeling.
However, because of the high content of Na2O (0.05 wt % or more), it does not have a sufficiently high electric resistance even though its glass phase is stable.
The refractory material proposed in Japanese Patent Laid-open No. Hei-10-59768 is a high-resistance high-zirconia electrocast refractory material which contains 0.05 wt % or more each of Na2O and K2O but does not contain alkaline earth metal oxides such as BaO. It is stable to repeated heating.
However, because of the lack of alkaline earth metal oxides, it needs to contain 0.05 wt % or more of Na2O to stabilize the glass phase. Consequently, it does not have a sufficiently high electric resistance.
The refractory material proposed in WO2005/068393 is a high-resistance high-zirconia electrocast refractory material which contains 0.8 wt % or more of Al2O3, less than 0.04 wt % of Na2O, and less than 0.4 wt % of CaO.
However, because of its high content of Al2O3 (0.8 wt % or more), it does not have a sufficiently high electric resistance.
Incorporation with CaO to stabilize the glass phase needs careful control because excess CaO promotes the formation of zircon.